Try: Charles Darwin
St Andrew's Street
Tel. (44) 1223 334900
(where 44 is the UK country code).
And for a valid card, then authorize.net has a list of cards here:
(but I am pretty sure you already knew that!!)
Authorize.net responded to me and do not have a set of test codes or matching addresses SPECIFIC to the UK.
Their more general listing of test codes for cards is above.
But submitting cards to their Gateway does not seem to be the issue.
When I personally test for myself (using UK addresses I know for a fact exist because I used to live in them!!) then when I fill in the initial checkout page the "Continue" button fails to transfer me to the terminating checkout page (which is where I would then fill in credit card information, that is then sent to authorize.net).
I.e.I am failing to get beyond the first of the two eCart checkout pages.
I have provided you with an existing UK address along with the required test card info for you to try for yourself.
Please do alert me, however, if you do get beyond the first checkout page and the continue button works for you, because I have to put authorize.net into test mode or the test card will fail.
(Or if you fix things and can get past the first page, I can run my own test).
But this is not yet anything to do with authorize.net or or with invalid card info, because a UK address is failing to get past page 1
My pages in private area